This invention relates to a reactor adapted to be mounted to a switching power supply and a powder core suitable for use in the reactor.
In recent years, energy saving and global warming resulting from the increase in CO2 emission are growing into important issues. In view of the above, energy saving technology is rapidly developed with respect to domestic electrical appliances and industrial apparatuses. Generally, motor-driven products, such as an air conditioner and a refrigerator, and lighting appliances are large in power consumption and are therefore given priority in development of the energy saving technology.
In order to improve the energy saving effect in those products, it is required to use a high-efficiency motor and to increase the efficiency of an electric circuit. In the electric circuit, the problem of efficiency resides in a power supply section for converting an a.c. input of 50/60 Hz into a d.c. output. In order to improve the efficiency, a switching power supply is recently and rapidly wide spread in the industrial apparatuses and the domestic electric appliances.
However, if the switching power supply is used, there arises a problem of generation of a harmonic current due to waveform distortion of electric current. In order to avoid the above-mentioned problem, proposal has been made of various circuit systems, for example, a choke input system, a single-transistor converter system, and an active filter system. In either system, a reactor is used to widen a conductive angle of the electric current.
Such reactor is required to have a wide variety of characteristics, such as a desired inductance value, a high conversion efficiency, no beat in an audible region, little temperature elevation, reduction in size and weight, and low cost. Various methods are available to individually and independently achieve each of the above-mentioned characteristics. On the other hand, in order to simultaneously and collectively achieve all of the above-mentioned characteristics, it is most effective to increase a switching frequency of the switching power supply to a high level, for example, 10 kHz or more. In this event, the reactor must be made of a material exhibiting low loss even at a relatively high frequency and having high permeability at a rated current.
In fact, it is well known that commercialization of a ferrite material for use at a high frequency greatly contributes to improvement of a small-capacity switching power supply operable at a high frequency as the switching frequency.
On the other hand, if the reactor is used in a large-capacity switching power supply, d.c. superposition characteristics are important in addition to the above-mentioned characteristics. Therefore, the ferrite material low in saturation magnetization can not be used but a different material must be used for the reactor. However, an ordinary silicon steel plate exhibits high core loss at a high frequency. Even a high silicon steel plate suffers a drastic increase in core loss and considerable deterioration in permeability at a frequency higher than 20 kHz. Therefore, a used frequency as the switching frequency is limited to 20 kHz or less. On the other hand, an amorphous material is high in cost because expensive boron is used and a special production facility is required. In addition, generation of beat in the audible region is inevitable because of large magnetostriction. Therefore, the amorphous material is not an optimum material.
In comparison, a powder core is excellent in frequency characteristics. As a disadvantage, an initial permeability is low. However, it is known that, by lowering the initial permeability, the powder core is excellent in d.c. superposition characteristics, specifically, in permeability at an applied d.c. magnetic field around 4000 A/m and that the core loss is relatively low. However, the d.c. superposition characteristics required for the reactor are under a high magnetic field around 12000 A/m. Furthermore, the core loss characteristic is also important. Therefore, the existing powder core can not satisfy the d.c. superposition characteristics as required for the reactor.
Generally, in order to improve the d.c. superposition characteristics, it is proposed to increase the saturation magnetization of the magnetic core and to form a gap at a part of a magnetic path. For example, Japanese Unexamined Patent Publication (A) No. H02-290002 discloses a powder core using Si-Fe alloy powder high in saturation magnetization. However, this publication merely describes the improvement of the initial permeability and the frequency characteristics and does not disclose the improvement of the d.c. superposition characteristics and the core loss characteristic at all. In the large-capacity switching power supply presently used, the switching frequency is limited to 20 kHz or less and a reactor comprising a magnetic core formed by laminating high silicon steel plates and a magnet wire wound around the magnetic core is used.
For the future, the energy saving and the suppression of CO2 emission are inevitable problems to be continuously dealt with. Therefore, the large-capacity switching power supply is inevitably and essentially required to be operable at a high frequency. As a consequence, there is a strong demand for a reactor adapted to such switching power supply.
It is therefore an object of this invention to provide a reactor adapted to a large-capacity switching power supply operable at a high frequency.
It is another object of this invention to provide a powder core which contributes to the achievement of the above-mentioned reactor.
It is still another object of this invention to provide a powder core capable of achieving the improvement of d.c. superposition characteristics under a high magnetic field and the reduction in core loss.
As a result of accumulation of studies upon the powder core for the reactor, the inventors have found optimum conditions for the composition and the properties of alloy powder used in the powder core and for a method of producing the powder core and hereby propose this invention.
According to this invention, there is provided a powder core which is obtained by preparing alloy powder comprising 3.0-8.0 wt % Si, 0.1-1.0 wt % O, 0-2.0 wt % (0 being exclusive) of at least one element selected from Mn, Al, V, Cr, and Ti, and balance Fe and having a particle size substantially equal to 150 xcexcm or less, mixing the alloy powder and a binder to form a mixture, and press-forming the mixture by the use of a die and which has a 20 kHz a.c. permeability equal to 20 or more under an applied d.c. magnetic field of 12000 A/m, a core loss characteristic of 1000 kW/m3 or less under the conditions of 20 kHz and 0.1 T, saturation magnetization of 10000 G or more, and coercive force of 3.0 Oe or less.
According to this invention, there is also provided a reactor comprising the above-mentioned powder core and a winding wound around the magnetic core.
In order to improve the d.c. superposition characteristics of the powder core, it is necessary to use a magnetic material having saturation magnetization as high as possible and exhibiting minimum variation in permeability in response to variation in magnetic field, i.e., exhibiting a flat magnetization curve. Such magnetic material may be a low Si-Fe alloy, Permalloy PB, or pure iron. In view of the characteristics and the cost, the magnetic material is generally limited to the low Si-Fe alloy.
The flat magnetization curve may be achieved by a technique of replacing a part of a magnetic path by a gap or a nonmagnetic material. However, since the powder core inherently has a low initial permeability, desired characteristics can not be achieved by the above-mentioned technique alone. The present inventors found out that an alloy lower in Si content than a 8.0% Si-Fe alloy and containing 0-2.0 wt % (0 being exclusive) of at least one element selected from Mn, Al, V, Cr, and Ti and 0.1-1.0 wt % O has a flat magnetization curve even under a high magnetic field and is therefore excellent in d.c. superposition characteristics.
This shows that magnetic anisotropy of an appropriate level is effective in improving the d.c. superposition characteristics. Presumably, the content of O has a certain effect in giving the magnetic material the magnetic anisotropy of an appropriate level. On the other hand, the ratio of C is preferably suppressed to 300 ppm or less because C has an effect of increasing the coercive force to cause core loss. With the above-mentioned structure, no special apparatus is required to produce the reactor. Therefore, the reactor can be provided in a simple process and at a low cost.
Herein, description will be made of the reasons why the composition of the alloy is defined as mentioned above.
If the content of Si is smaller than 3.0 wt %, the alloy has high magnetic anisotropy and low resistivity which results in an increase in core loss. If the content of Si is greater than 8.0 wt %, the alloy has low saturation magnetization and high hardness which lowers the density of the powder that is press-formed to form a compact body. As a result, the d.c. superposition characteristics are deteriorated.
If the content of O is smaller than 0.1 wt %, the initial permeability is excessively high so that the d.c. superposition characteristics are not improved. If the content of O is greater than 1.0 wt %, the ratio of the magnetic material in the powder is decreased so that the saturation magnetization is considerably degraded. As a result, the d.c. superposition characteristics are deteriorated.
By addition of the additive or additives selected from Mn, Al, V, Cr, and Ti, the magnetic properties are improved. However, if the total amount of the additive or additives is greater than 2.0 wt %, the saturation magnetization is remarkably decreased. As a result, the d.c. superposition characteristics are deteriorated.
If the content of the binder falls within a range greater than 3.0 wt, the powder core has a low powder packing fraction so that the saturation magnetization is lowered. In view of the above, the binder is preferably mixed at a ratio of 3 wt % or less. After compaction-forming, the powder core is subjected to heat treatment for removing the distortion. Therefore, it is preferable to use silicone resin as the binder.